So far, a lot of layered compounds have been developed by using layered compounds such as clay minerals to include a variety of cation or cationic functional organic materials in them. This has made use of the fact that layers of clay mineral have negative charges so that interlayer cations are susceptible of ion exchange. Unlike clay minerals, LDHs have positive charges on layers and anions between layers, and so have anion exchangeability. Typical of naturally occurring minerals is hydrotalcite (Mg3Al(OH)8(CO32−)0.5.2H2O). Inorganic anion exchangeable materials are much more limited in type than cation exchangeable compounds, and LDHs have attracted attention in this respect (Non-Patent Publication 1).
LDHs inclusive of hydrotalcite often contain carbonate ions between layers, primarily because carbonate ions have much stronger affinity for LDHs than other anions (Non-Patent Publication 2). For this reason, an LDH (of the carbonate ion type) containing carbonate ions is very low in ion exchangeability, and anion exchange hardly takes place under ordinary conditions: it has very limited use as an anion exchange agent.
Generally, an LDH is synthesized by coprecipitation reactions wherein aqueous solutions of a magnesium salt and an aluminum salt are mixed together while an aqueous solution of sodium hydroxide is kept at a pH of about 10. In this case, if anions other than carbonate ions (for instance, nitrate or chlorine ions) are allowed to coexist, it is then possible to synthesize LDHs containing easy-to-exchange anions. However, the reactions involved must be carried out using carbon dioxide-free, degassed distilled water or an alkali reagent in a carbon dioxide-free atmosphere such as a nitrogen stream. Steps of separation by filtration and drying, too, must be all implemented in a carbon dioxide-free atmosphere. Further, because synthesis conditions vary depending on the type of anions, there is much difficulty in the optimization of synthesis conditions and indeed, LDHs synthesized of any other type are found to be poor in crystallinity (Non-Patent Publication 3).
Recently, LDHs having uniform grain diameter and large yet controlled crystal diameter have been synthesized by carrying out uniform nucleation using a reagent that emits out ammonia in hot water, especially urea, and hexamethylenetetramine. However, the LDHs obtained in this case are only of the carbonate ion type (Non-Patent Publications 4, 5 and 6).
There is also an alternative method harnessing structural changes in association with heating. More specifically, the application of heat of about 500° C. causes hydrotalcite to change in structure for decarbonation; however, it has been known that if the obtained product is put in an aqueous solution containing anions other than carbonate ions (e.g., nitrate ions, and chlorine ions), an LDH containing those anions is then reconstructed (Non-Patent Publication 7). Using this “reconstruction”, any desired anions are included between layers for synthesis of a nano-layered compound in which functional molecules or organic materials are included between layers at a nano-level. Due to structural changes caused by heating, however, the reconstructed product is far away from the starting material, leading to problems such as decreased crystallinity after repeated reconstruction (Non-Patent Publication 8), and changes in terms of crystal shape, grain diameter, homogeneity, and composition (Non-Patent Publication 9). Decarbonation requires a several-hour heating at temperatures as high as 500° C., which would be impractical from both energy and time considerations.
The carbonate ion type is easy to synthesize and indeed, most of the industrially produced LDHs are of this carbonate ion type. If the LDHs of the carbonate ion type can be converted by a simple chemical method into anion exchangeable LDHs that contain easy-to-ion exchange anions (e.g., nitrate ions, and chlorine ions), then they could be extended to a wide of applications industrially as well as at research and testing levels.
For instance, one LDH application is to entrap carbon dioxide (CO2). Carbon dioxide is a grave factor responsible for global warming, and entrapping CO2 emissions is of great importance as well known in the art. Having been studied so far for the purpose of entrapping CO2, LDHs are typical of inorganic CO2-fixation materials. For entrapping of CO2, there is a product used which is obtained from hydrotalcite structurally changed and decarbonated by heating of the order of 500° C. As already mentioned, this makes use of the “reconstruction” phenomenon, but does not gain popularity as an effective process for the reason of demerits in consideration of energy and time, because decarbonation requires high temperatures. LDHs of the carbonate ion type occurring by reactions with CO2 cannot be recycled unless again decarbonated by heating of the order of 500° C.; they have been considered as only a fixation material chiefly for fixing and throwing away CO2.
For repeated separation and recovery of CO2 instead of such fixation that does not allow for recycling, for instance, a chemical absorption method using alkanolamines is now under study. However, the gravest problem with that approach was that a temperature on the order of 100° C. is needed for recovery of CO2 (Non-Patent Publication 11). If this decarbonation is achievable under mild conditions such as room temperature and by a simple chemical method, and if an LDH containing easy-to-anion exchange anions (e.g., nitrate ions, and chlorine ions) is obtained by conversion for repeated use, it could provide a very promise candidate for materials capable of separating off and recovering CO2.
Another possible application of LDHs is that their anion exchangeability is used for separation and recovery of harmful or useful anions from in water. As already reported in the art, anion exchangeability has attracted attention and been studied about affinity order (Non-Patent Publication 2). As a matter of course, this may be used to entrap harmful or useful anions in water and indeed, how to remove phosphorus components from in water using LDHs has been proposed. Referring to regeneration of LDHs turning into the carbonate ion type, however, there has been only an inefficient process proposed, in which after “reconstruction” relying upon heating of LDH on the order of 500° C. or complete dissolution of LDH, that LDH is again synthesized by “coprecipitation” (Patent Publication 1). If the LDH can be readily converted into an easy-to-anion exchange LDH, it is then possible to separate off and recover harmful or useful anions from water: these anions trapped in the LDH can be treated with carbonate ions for recovery. And the LDH changed into the carbonate ion type is regenerated by a simple de-carbonation operation into an easy-to-anion exchange LDH for repeated use, which would be a very promising candidate for materials capable of separating off and recovering harmful or useful anions.
Yet another possible LDH application is filler that can make improvements in the mechanical strength of general-purpose polymers. Two-dimensional layer-containing compounds such as clay minerals are used for the filler, but there are still problems such as incomplete exfoliation of layers, and an insufficient affinity for matrixes; there is still much left to be desired in terms of dispersibility, etc. Unlike clay minerals, LDH's layers are so of cationic nature that anionic monomers, for which clay does not work, can be included in them. If the monomers can be included between layers to trigger reactions, it then may be applied to high molecules for which clay minerals cannot be used, and may possibly be extended to even foliation of layers, resulting in some considerable improvements in mechanical strength, gas barrier properties, etc. (Non-Patent Publications 12 and 13).
Thus, if a carbonate ion type LDH can be converted by a simple chemical method into an easy-to-anion exchange LDH, it could then proffer breakthroughs from a lab level to an industrial level as well as in a wider range from environmental problems to nanotechnologies (Non-Patent Publication 14).
We have already discovered that upon action of a hydrochloric acid/NaCl (sodium chloride) mixed solution on a carbonate ion type LDH at room temperature, de-carbonation occurs, resulting in conversion into an LDH containing the added anions, and filed a patent application for it. XRD diffraction, observation under SEMs (scanning electron microscopes), and gravimetric analysis has revealed that if this decarbonation method is implemented under the optimized conditions, crystal shape, crystal structure and crystallinity are kept intact with no or little weight change due to dissolution (Non-Patent Publications 15 and 16, and Patent Publication 2).
However, this method using a hydrochloric acid/NaCl mixed solution has some problems stemming from the fact that the solution is of strong acidity. Originally, LDHs are vulnerable to acids, and they dissolve by themselves in a low pH (hydrogen ion exponent) state (of weak acidity). In actual measurements, LDHs dissolve visibly in a pH state of less than 4 upon only a few-hour contact with the solution. When it comes to the most general anion exchange process, for instance, a column in which an anion exchange material is filled in a continuous manner, it is not always in uniform contact with the solution. In industrial applications where mass processing is in need and a continuous rather than batch process is in need, it is important to keep the pH as close to neutrality as possible, and keep the pH against changes due to reactions. It goes without saying that a solution of pH=1 to 2 is of so strong acidity that meticulous care should be taken of handling, and there is a corrosion problem as well with equipments for using volatile acids such as hydrochloric acid.
Thus, the decarbonation method relying on a hydrochloric acid/NaCl mixed solution provides an epoch-making method for synthesizing an LDH containing easy-to-ion exchange anions (e.g., nitrate ions, and chlorine ions) by a simple chemical method yet without affecting grain diameter and homogeneity, but there are still problems stemming from the fact that there is an aqueous solution of strong acidity used, and to allow it to be used in a wider range of industrial applications, much more improvements must be made.
Listing of the Patent, and Non-Patent Publications
    Patent Publication 1: JP(A) 2001-187336    Patent Publication 2: JP(A) 2005-255441    Patent Publication 3: JP(A) 2007-31189 (see paragraph [0052])    Non-Patent Publication 1: Cavani, F., Trifiro, F., Vaccari, A., Catal. Today 11, 173-301 (991)    Non-Patent Publication 2: Miyata, S., Clays Clay Miner. 31, 305-311 (1983)    Non-Patent Publication 3: Reichle, W. T., Solid States Ionics 22, 135-141 (1986)    Non-Patent Publication 4: Costantino, U., Marmottini, F., Nochetti, M., Vivani, R., Eur. J. Inorg. Chem. 1439-1446 (1998)    Non-Patent Publication 5: Ogawa, M., Kaiho, H., Langmuir 18, 4240-4242 (2002)    Non-Patent Publication 6: Iyi, N., Matsumoto, T., Kaneko, Y., Kitamura, K., chem. Lett. 33, 1122-1123 (2004)    Non-Patent Publication 7: Hibino, T., Yamashita, Y., Kosuge, K., Caly Clay Miner. 43, 427-432 (1995)    Non-Patent Publication 8: Hibino, T., Tsunashima, A., Chem. Mater. 10, 4055-4061 (1998)    Non-Patent Publication 9: Stanimirova, T. S., Kirov, G., Dinolova, E., J. Mater. Sci. Lett. 20, 453-455 (200)    Non-Patent Publication 10: Suzuki, M. “Isolation of CO2 by Minerals” (“State of the Art for Fixation and Isolation of CO2” edited by Inui, T., pp. 124-135), CMC (2000)    Non-Patent Publication 11: Matsumoto, K., “Carbonic Acid Gas Separation Technology by Chemical Adsorption” (“State of the Art for Fixation and Isolation of CO2” edited by Inui, T., pp. 87-97), CMC (2000)    Non-Patent Publication 12: Leroux, F.; Besse, J.-P., Chem. Mater. 13, 3507-3515 (2001)    Non-Patent Publication 13: Newman, S. P., Jones, W., New J. Chem. 22, 105-115 (1998)    Non-Patent Publication 14: Khan, A. I., O'Hare, D. J., J. Mat. Chem. 12, 3191-3198 (2002)    Non-Patent Publication 15: Iyi, N., Matsumoto, T., Kaneko, Y., Kitamura, K., Chem. Mater. 16, 2926-2932 (2004)    Non-Patent Publication 16: Iyi, N., Okamoto, K., Kaneko, Y., Matsumoto, T., Chem. Lett. 34, 932-933 (2005)    Non-Patent Publication 17: Okamoto, K., Sasaki, T., Fujita, T., Iyi, N., J. Mater. Chem., 16, 1608-1616 (2006) (see paragraph [0052])